Cross-shelf life-stage segregation and community structure of the euphasiids off central Oregon (1979-1972)

Abstract

Community structure and zonal distribution of euphausiids along the Newport Hydrographic line (44°39.1'N) off central Oregon, USA, were examined from bi-weekly oceanographic surveys (n=48) carried out from January 1970 through July 1972. We explored the associations among the euphausiid community composition (14 species) and changes in sea-surface temperature (SST), sea-surface salinity (SSS), coastal upwelling index (CUI), sigma-t, day vs. night sampling time, and distance from the coast. This period included the 1970-1971 La Niña and, at low latitudes, the 1972 El Niño events; this study shows that this El Niño event did not reach the Oregon coast as have most other most recent events. Multivariate analyses were done to identify the cross-shelf environmental gradients that most influence temporal changes in the euphausiid community structure. Non-metric multidimensional scaling (NMDS) indicated that the euphausiid community separates into coastal and oceanic assemblages, with a species-richness gradient located about 45 km from shore, over the shelf-break. In the inshore zone, positive CUI (upwelling) was associated with Thysanoessa spinifera, while negative CUI (downwelling) was associated with the presence of the oceanic species assemblage during October-November. Indicator species analysis (ISA) showed that two species were good indicators for the oceanic environment: Thysanoessa longipes and Nematoscelis atlantica. Three species (Thysanoessa gregaria, Nematoscelis difficilis, and Tessarabrachion oculatum) were moderate indicators for the oceanic environment because, during fall and winter downwelling events, they may be transported over the continental shelf. High densities of larvae and juveniles of T. spinifera were found nearshore (<18 km from the coast), but older stages were mainly recorded offshore (>18-108 km from the coast). Euphausia pacifica was relatively homogeneously distributed in shelf and offshore waters, but its larvae were recorded chiefly inshore in mid-summer and offshore during the rest of the year. Our data showed a strong cross-shelf gradient in euphausiid assemblages and life-stage-segregated distributions for T. spinifera and E. pacifica, suggesting active population maintenance strategies within this upwelling region.